In a work barge such as a tugboat, since a main engine is selected according to the output required in work, the load factor of the main engine is lowered in the moving or standby state except the working state, and this is unfavorable in the fuel cost and the maintenance and management. Further, when being on standby in the offing, moving at low speed is needed to keep a position according to the sea meteoric conditions. However, since the work barge mostly uses a fixed pitch propeller, in the case where the number of revolutions of the main engine is lower than the idling number of revolutions, the speed is controlled by the slip control of a clutch. Consequently, output loss and heating loss due to a slip occur to lower the efficiency, and further the emission amount of greenhouse gas is larger as compared with the obtained output.
On the other hand, in the variable speed control by electric propulsion which has been increasing in recent years, the slip control in the low speed moving state is not required, but the work barge such as a tugboat requires great output in work, so that when transmission loss due to electric propulsion is taken into consideration, the required capacity of a power generation engine and output of the motor are increased, resulting in the problem of difficulty in casing the engine and the motor in the present ship form.
In order to address such a problem, a hybrid type marine propulsion device utilizing both the driving force of a main engine and the driving force of a motor has been proposed.
The marine propulsion device disclosed in Patent Literature 1 is, for instance, a kind of the so-called conventional type marine propulsion device in which a main engine, a speed reducer and a propeller shaft are arranged in a line, and the driving force of the main engine drives a propeller through the speed reducer. This marine propulsion device has a speed reducing reverse gear provided in the interior with a complicated speed reducing and reversing mechanism including a clutch, a gear and the like, in which a propeller is driven by a prime mover and one of a motor and a motor generator or both thereof, so that the output of the prime mover is efficiently used, and the emission of exhaust gas is decreased to realize low vibration and low noise.
On the other hand, the marine propulsion device disclosed in Patent Literature 2 has a configuration in which a main engine is connected through a clutch to a propulsion device including a propeller and a speed reducer, and also an electric propulsion motor is direct-coupled thereto, and further a generator is direct-coupled to the opposite side to the propulsion device of the main engine, so that during high-speed navigation, the propulsion device is driven only by the main engine, and during low-speed navigation, the propulsion device is driven only by the electric propulsion motor and the generator is driven by the main engine. Thus, it is possible to provide a marine hybrid propulsion system, which may realize high function of the main engine driving generator and miniaturization of the motor for auxiliary machinery, and remarkably reduce the equipment cost.